The present invention relates to carburetor control systems for internal combustion engines and, more particularly, to such systems for small internal combustion engines which incorporate a speed regulating governor to maintain the speed of the engine relatively constant under different loading conditions.
Small internal combustion engines are used in a variety of applications, including lawnmowers, snowblowers, and engine-alternator sets, to drive a variable load at a controlled operator selected speed setting. For instance, in the use of a lawnmower powered by an internal combustion engine, it is desired that the selected speed of the engine remain relatively constant under a variety of loading conditions. Thus, whether the lawnmower encounters tall grass or short grass, the lawnmower speed which has been selected by the operator should remain constant. Likewise, in the case of an engine-alternator set, it is essential that the alternator output frequency, i.e., the engine drive speed, remain constant despite changes in the electrical loads connected to the alternator output.
In the conventional design of mechanical speed regulating governors for small internal combustion engines, a throttle valve is mechanically linked to a governor lever which is acted upon by opposing forces representing the desired engine speed and the actual engine speed. The force representing the desired engine speed is typically provided by a spring linkage between the governor lever and a manually operable control lever. The opposing force representing the actual engine speed is provided by either an air vane mechanism or a centrifugal flyweight mechanism sensitive to the engine speed. Controlled movement of the throttle valve in response to a change in the desired engine speed setting or the engine load is proportional to the difference between the desired speed and the actual engine speed. Accordingly, the mechanical speed regulating governors of the prior art exhibit proportional control of the engine speed.
A problem arises in the mechanical speed regulating governors of the prior art, in that proportional control of engine speed results in offset error, or "droop", which reduces the governed engine speed when a load is applied and also manifests itself following a change in the desired engine speed setting. For example, if an engine is operating at steady state at a given desired speed under a constant load, and then the desired speed setting is increased by a desired amount, the proportional controller is not able to increase the actual speed of the engine by the desired amount, but instead increases the engine speed by something less than the desired amount. Likewise, when an engine operating at steady state at a given desired speed experiences a positive load change, thereby reducing the engine speed, a proportional controller is not capable of restoring the engine speed to the original desired speed, but rather to some lesser speed due to offset. In some applications, the problem of "droop" is not critical because the operator may either accept the lower speed or compensate by further increasing the desired speed setting. However, in an automatic, constant speed application such as an engine-alternator set, the problem of offset is less tolerable.
While the problem of offset in proportional controllers can be minimized by increasing the gain of the controller, this solution is unsatisfactory because high gain will cause an unstable process. Accordingly, it is desired to provide a controller which responds initially with small gain to achieve stable control, and then changes to high gain to overcome offset. Such a technique is practiced in a proportional plus integral controller, wherein integral control consists of the time integral of the difference between the desired speed setting and the actual speed.
A speed regulating arrangement for an internal combustion engine is disclosed in U.S. Pat. No. 3,800,755, issued to Klaiber et al, wherein proportional plus integral control of an engine is accomplished by actuation of the engine's throttle valve by an electromagnet having a rotatable armature. The electromagnet is controlled by an electronic speed regulating circuit having a regulating stage which exhibits both proportional and integral feedback. Several problems are associated with the aforementioned electronic control of an internal combustion engine, including the inability of the engine to operate in the event of a failure of any of the system's components. Also, electromagnetic actuators having the necessary linear response characteristics to perform proportional plus integral control are relatively expensive and require considerable power, thereby adding to the expense of the associated control circuitry.
Accordingly, it is desired to provide a speed regulating governor for an internal combustion engine which overcomes the problems and disadvantages of the prior mechanical proportional control systems and electronic proportional plus integral control systems.